Allergy shots, formally known as subcutaneous immunotherapy (SCIT), are a long-term treatment designed to reduce the severity of allergic reactions. This therapy works by introducing small, controlled amounts of specific allergens over time to desensitize the immune system. The goal is to train the immune system to tolerate these substances rather than overreacting to them. Since the treatment involves injecting the substance that causes the reaction, the possibility of an adverse event exists. Understanding the time frame in which reactions can occur is fundamental for patient safety and is a standard part of the treatment protocol.
The Critical Monitoring Period: Immediate Reactions
The most serious adverse event is a systemic reaction, which affects multiple body systems and can lead to anaphylaxis. Most of these events happen very quickly because the allergen extract is absorbed into the bloodstream from the subcutaneous fat layer. Studies have shown that the vast majority of systemic reactions—approximately 85%—begin within 30 minutes of the injection.
This rapid onset is linked to the speed at which the body absorbs the injected material. To mitigate the risk of a life-threatening event outside a clinical setting, patients are required to remain in the clinic for at least 30 minutes after receiving their shot. This mandatory observation period allows medical staff to promptly recognize and treat any severe reaction with emergency medication.
A higher risk of immediate reaction can occur if the injection is inadvertently administered too deeply into muscle tissue instead of the intended subcutaneous layer. The goal of a subcutaneous injection is to create a small reservoir of the allergen that is absorbed slowly over time. Certain patient factors, such as having uncontrolled asthma, can also increase the risk during this immediate window.
Understanding Delayed or Local Reactions
Reactions to allergy shots are not always immediate and can manifest hours after the patient has left the clinic. The most common type of delayed event is a local reaction, confined to the injection site. These reactions involve swelling, redness, itching, or irritation at the injection point and occur in a significant number of patients (26% to 86%).
These localized events are considered mild and represent a robust, localized immune response to the allergen dose. Local reactions typically begin within a few hours of the injection. They tend to peak in severity around 6 to 12 hours post-injection, and usually resolve completely within 24 to 72 hours.
Systemic reactions can also be delayed, occurring more than 30 minutes after the shot. Approximately 14% of all systemic reactions fall into this delayed-onset category. These events can manifest as widespread hives or other systemic symptoms hours after the patient leaves the office, sometimes up to six hours later. These delayed systemic reactions still require immediate attention, as they reflect a generalized immune response.
Recognizing Symptoms and Emergency Response
Distinguishing between a mild local reaction and a more serious systemic event is crucial for safe allergy shot treatment. A mild local reaction is limited to the injection site and may involve swelling larger than a quarter, but without any symptoms elsewhere in the body. This type of reaction is usually managed easily at home with a cold compress or an over-the-counter oral antihistamine.
Signs of a severe systemic reaction, even if they appear hours later, require immediate emergency action. These symptoms include a sudden onset of widespread hives, generalized itching, or flushing. More severe signs involve the respiratory or cardiovascular systems, such as wheezing, difficulty breathing, throat tightness, or a sudden drop in blood pressure causing dizziness or fainting.
For any suspected severe or systemic reaction, the protocol requires the immediate use of an epinephrine auto-injector, commonly known as an EpiPen. Epinephrine is the medication of choice for anaphylaxis and must be administered first, followed by calling for emergency medical services. Patients must communicate any reaction, whether immediate or delayed, to their allergist before their next scheduled dose, as the treatment plan may need adjustment to ensure continued safety.